1. Field of the Invention
This invention relates to the preparation of polymer foams and is more particularly concerned with novel catalyst combinations for the trimerization of polyisocyanates to polyisocyanurates and their utilization in the preparation of polyisocyanurate foams.
2. Description of the Prior Art
Rigid polyisocyanurate foams having high resistance to flame and heat as well as excellent thermal insulating capacity are known in the art. The prior art discloses methods for preparing such foams by reacting an organic polyisocyanate with a trimerizing catalyst in the presence of a blowing agent, and a minor proportion (usually less than 0.5 equivalent per equivalent of polyisocyanate) of a polyol; see for example U. S. Pats. No. 3,516,950, 3,580,868, 3,620,986, 3,625,872, and 3,725,319. The process described in U. S. Pat. No. 3,745,133 discloses the use of a combination of an epoxide and a tertiary amine as cocatalysts.
It will be recognized by one skilled in the art that the method referred to hereinabove requires the foaming process to proceed in conjunction with at least two polymer forming reactions; namely, the isocyanurate formation arising from the homopolymerization of the isocyanate employed, and the minor amount of polyurethane formation arising from the reaction of a polyol with the isocyanate. Difficulties arise, particularly in commercial applications due to the variation in the rates of the two polymerization reactions. The polyurethane formation initiates before the trimerization formation thus giving two separate rise steps in the overall foam rise profile.
These difficulties are accentuated in the case of the preparation of polyisocyanurate foam laminate board stock. The preparation of the latter requires a foam rise profile which is both rapid in rise time, and cures at a rate, sufficient to provide good adhesion to the facing materials which form an integral part of the laminate board. However, because of the critical role which the nature and proportion of catalyst plays in the preparation of polyisocyanurate foams of the type referred to hereinabove, it has not hitherto been possible to prepare polyisocyanurate foam laminate board stock possessing optimum properties of adhesion, physical strength, and in particular, low volume change upon exposure to elevated temperatures. This latter property is of particular importance to the end use application of polyisocyanurate foam laminate board stock wherein the laminate is being subjected to extremes in temperature, such as in roof decking.
We have now found a catalyst system which is free from the above difficulties and which is particularly useful in the preparation of foam laminates.